The positively reinforcing sequelae of opiate administration form the basis of the addiction process. Clinical data indicate, futhermore, that morphine and related drugs can produce euphoric effects. Since morphine enhances self-stimulation, it has been hypothesized that changes in the excitability of the medial forebrain bundle ("reward") system could form the neurophysiological basis of the addiction process. Experiments have been designed 1) to determine whether the opioids, and other drugs with high addiction liability, produce increases in "reward" system excitability (expressed as increased self- stimulation rates), and 2) to determine whether the influence of such drugs on self-stimulation can be used as a model for studying the addiction process.